1. The Field of the Invention
The present invention relates to apparatus and process for adjustment for the position of the slice lip of the headbox of a paper making machine.
2. State of the Art
The headbox of a paper making machine converts the flow of a fibrous suspension, forming a paper slurry, from a stream into a layer corresponding to the width of the sheet to be formed. The headbox takes the form of a reservoir, the front face of which has, towards the bottom, a slot provided with lips, between which the liquid slurry is projected onto a production wire. The purpose of the headbox is to insure a constant delivery and to distribute the slurry over the entire width of the wire, which may be as much as 9 meters in the case of modern installations.
The slurry containing fibres in suspension as well as mineral filler is forced under a constant pressure between the lips. The evenness of the rate of discharge of a layer containing fibres in suspension together with the uniformity of the concentration determine the uniformity of the weight per unit area of the manufactured product. In modern paper making machines, flow takes place between metalic lips, one of which is fixed while the other is movable as a whole so that it adjusts the thickness. Furthermore, the movable lip, which is usually the upper lip, is deformable lengthwise under the action of slice rods which can be moved to control the position of various parts of the slice lip. Conventionally, slice rods are controlled by manually-operated screw jacks. A modern normally includes several dozen of such manually operated screw jacks.
Control of the thickness of the layer of the slurry as it passes through the lips and thus control of weight per unit area of the product at the end of the machine is not very conveniently carried out with the aid of such manually operated screw jacks, the adjustment of which is a delicate matter which gives good results only on a trial and error basis. To eliminate this disadvantage, headboxes have been designed wherein the screwjacks are each controlled by an electric motor-reducer unit which is itself controlled, by way of a computer which receives results of the measurements of paper thickness or basis weight and calculates the corrections to be made on the screw jacks.
These control systems using motor-reducers are not completely satisfactory. In fact, mechanical backlash generally occurs, and precision of adjustment is not very great. However, the greatest drawback is the very high cost of these mechanisms, since the headbox of a modern machine may comprise up to 60 or more of them. Moreover, electrical components can be subject to corrosion.
In addition to motor-reducers, hydraulic means have also been used to control the position of of slice lip. U.S. Pat. No. 2,951,007 describes a hydraulic system for adjusting the movable lip of a headbox. It is apparent from the patent that the hydraulic system for operating the slice lip must be installed on a paper machine headbox when the headbox is originally built. It appears that a headbox having a manually operated screwjack could not be converted to a hydraulically-operated system described in the patent without practically completely rebuilding the headbox.